1. Technical Field
The invention relates to optical waveguides and photonic circuits. In particular, the invention relates to optical waveguide resonators used in photonic devices and systems.
2. Description of Related Art
An optical ring resonator generally comprises a segment or segments of optical waveguide arranged as a closed loop or ring. The closed loop forms a resonant cavity. Such ring resonators have a variety of important uses in photonic devices and systems. In particular, the optical ring resonator may serve as an active photonic device having one or more active optical components or elements integrated within the resonant cavity of the ring resonator (i.e., an ‘intracavity’ active element(s)). The intracavity active element(s) integrated with the ring resonator may result in ‘cavity enhancement’ of a functionality of the active elements, for example. Such intracavity active elements may be employed to one or more of generate, amplify, and modulate an optical signal of the ring resonator.
The intracavity active elements that are integrated with a ring resonator may be generally classified as either saturable active elements or unsaturable active elements. Saturable active elements include, but are not limited to saturable absorbers, laser and other optical amplifiers. For example, the saturable active element may be a continuous-wave laser amplifier or a directly-modulated laser amplifier that is modulated by an applied electric current. In another example, the intracavity active elements may comprise a laser amplifier in combination with a saturable absorber. The saturable absorber may assist in mode-locking the laser within the resonant cavity (i.e., laser cavity) formed by the ring resonator, for example. An example of an unsaturable active element is an electro-absorption modulator (EAM). Incorporated into the resonant cavity of the ring resonator, the EAM becomes a ‘cavity enhanced’ EAM.
Among the challenges of integrating an intracavity active element into an optical ring resonator is providing sufficient coupling between the intracavity active optical element and a mode or modes of the optical signal propagating in the optical waveguide of the ring resonator. Generally, the coupling must be sufficient to provide for proper and/or efficient operation of the intracavity active optical element. For example, a modulation depth or ratio of an EAM is directly dependent on a percentage of the optical signal that is coupled into the EAM. Sufficient coupling may be difficult to achieve in many cases.
Another challenge of integrating an intracavity active optical element into an optical ring resonator is choosing appropriate materials for the integrated structure. Specifically, an ideal or highly desirable material for realizing the optical waveguide may be less than ideal for or even incompatible with an implementation of the intracavity active element. For example, while silicon may be a relatively attractive material (e.g., low cost) for realizing and implementing the optical waveguide of the optical resonator, silicon is an indirect band gap material and, as such, is not particularly well-suited for constructing active optical elements.